Putative eicosanoid biosynthesis pathway in Daphnia based on bioinformatic and transcriptomic evidence from D. pulex and D. magna. All the putative genes (names in italics) have been identified through different gene models in the D. pulex genome and are shown in black or grey font based on high (> 60%) or low (< 60%) similarity to proteins from other genomes (Table 1). Eicosanoids in grey font are less likely to be present in daphnids. Expression of ortholog genes in ibuprofen-stressed D. magna (24 h exposure to 20–80 mg l-1) was analysed using real-time quantitative PCR . Fold change difference in gene expression (mean ± SE) is shown relative to controls (grey values are only weakly significant). The enzyme LTB4DH (encoded by Ltb4dh), that catabolises PGE2, PGF2α and LTB4 to become inactive eicosanoids, is also known as 15-oxo-prostaglandin 13-reductase. All the specified eicosanoids have been identified in other arthropod species [1, 2], expect for the leukotrienes where only indirect evidence exits . Abbreviations: HETE, hydroxyeicosatetraenoic; HPETE, hydroperoxyeicosatetraenoic acid. See text for further details.